1. Field of the Invention
The present invention relates to ring laser gyroscopes and more particularly pertains to laser gyroscopes which are dithered to reduce the effects of mode locking.
2. Description of the Prior Art
In a laser gyroscope, a pair of counter-rotating light beams are propagated about a closed-loop path. The two beams traveling in opposite directions are compared in frequency. When the gyroscope rotates in inertial space, the two oppositely traveling beams see apparently unequal paths. This causes the laser to resonate at different frequencies for the two beams. The frequency difference may be measured as an indication of the direction and rate of the rotation.
For small rotation rates, however, mode or frequency locking occurs. The frequencies of the two oppositely-directed light beams lock to each other and the beams resonate together. This creates a nonlinear dead zone in the characteristic of the gyro for which no useful output can be obtained.
Among the schemes used to avoid or reduce the effect of the lock-in phenomenon is mechanical body dithering of the gyro. The entire gyro body is mechanically oscillated at a relatively high frequency. This gives the gyro a dynamically varying bias rotation rate which exceeds the threshold input rate or natural lock rate of the dead zone most of the time. This mechanical dither approach has been used successfully. However, it adds undesirable energy consumption, cost, weight, size and mechanical complexity to an attitude reference system employing it. Mechanical dithering also introduces an undesirable source of vibration and noise into a system. This vibration may cause problems in the use of other instruments, such as accelerometers, which may be mounted with the gyro on the same apparatus. An example of a ring laser gyro employing mechanical dither is given in Hutchings et al., "Counterbalanced Oscillating Ring Laser Gyro", U.S. Pat. No. 4,115,004 issued Sept. 19, 1978.
Optically active devices such as, for example, Faraday cells have been inserted into laser cavities to introdue a bias or asymmetry into the operation of the laser to thereby avoid mode locking. An example of such an approach is given in Henry, "Ferrimagnetic Faraday Elements for Ring Lasers", U.S. Pat. No. 4,222,668 issued Sept. 16, 1980, and assigned to Rockwell International Corporation, the assignee herein. Another example, using a birefringent optical element, appears in Newburgh, "Ring Laser Utilizing An Optical Retardation Plate to Prevent Beam Locking", U.S. Pat. No. 3,791,738 issued Feb. 12, 1974. In Newburgh, a varying Fresnel phase velocity shift is obtained by dithering the optical element.
In Henry, "Ring Laser Having Magnetic Isolation of Counter-Propagating Light Waves", U.S. Pat. No. 4,219,275 issued Aug. 26, 1980, and assigned to Rockwell International Corporation, the assignee herein, an optically active device inserted into the laser cavity introduces either a spatial or temporal separation of the counter-propagating light beams. This approach is intended to eliminate mode locking altogether. An approach to simultaneous temporal and spatial separation of the beams using shutters is disclosed in Chodorow, "Means for Avoiding Locking in Ring Lasers", U.S. Pat. No. 3,627,422 issued Dec. 14, 1971.
A problem arises in the approaches to unlocking gyros which insert optical elements in the laser cavity such as the approaches described by Henry, Newburgh, and Chodorow. The optical elements increase the number of interfaces between dissimilar materials in the optical path. Each such interface tends to be a source of increased scattering. The increased scattering tends to enhance the very mode locking effects which are intended to be reduced. Furthermore, the optical elements themselves introduce bias errors into the operation of the gyro due to such factors as, for example, asymmetry in their characteristics and random variations in their characteristics due, for example, to variations in temperature.
Electrical excitation, or pumping power, for lasing is typically applied to the laser gaseous medium from a direct current supply. A voltage sufficient to sustain a discharge in the medium is applied to spaced-apart electrodes. The discharge, or plasma excitation current, comprises a flow of ionized gas between oppositely polarized electrodes in the optical cavity. It has long been known that this plasma flow tends to induce a bias in a ring laser gyro. Therefore, laser gyros which use direct current excitation are typically provided with a balanced electrode structure wherein two electrodes of one polarity are symmetrically disposed about a single electrode of the opposite polarity. With this structure, two equal and opposite plasma excitation currents are caused to flow and their biases are thereby caused to cancel.
It is known that the electrical excitation to sustain the plasma excitation currents may be obtained from a supply which varies the voltage in such a way as to dynamically unbalance those currents. A dynamically varying bias in the gyro output, i.e., dither, may thereby be induced. However, the maximum amount of dither obtainable by the use of dynamically varying unbalance in the plasma excitation currents is too small for this approach to be of practical interest.
A scheme for biasing a ring laser gyro which involves pumping the plasma more vigorously is disclosed in Podgorski, "Externally Biased Ring Laser", U.S. Pat. No. 3,744,908 issued July 10, 1973. A linear induction motor positioned adjacent to the laser gain section pumps the ionized gas unidirectionally. The apparent index of refraction for the two oppositely traveling beams is thereby made different by an amount sufficient to avoid lock-in. This scheme is believed to use large amounts of energy. The requirement for an electromagnet structure increases the weight. In addition, in a scheme such as this where a unidirectional bias is induced, no cancellation of systematic errors occurs.